1. Field of the Invention
The present invention relates to a light deflecting system and, more specifically, to a rotary mirror system for angularly deflecting a light beam, for example, in laser printers for deflecting the laser beam in the main scanning direction.
2. Discussion of Related Art
A rotary mirror system of the type for deflecting light beams is described in JP-A-1-285917. In this known system, the mirror body is shaped as a polygonal disc with mirror surfaces provided at the outer circumferential edge surfaces of the disc. The disc is provided with magnets which co-operate with magnets of a stator, thereby constituting an electric motor for driving the disc. The disc is rotatably supported on the stator by means of an air bearing which is formed by two hemispherical members provided at the center of the disc on the opposing main surfaces thereof and received in mating hemispherical cavities formed in bearing members of the stator. The hemispherical bearing surfaces are provided with spiral grooves for introducing air into the narrow gap between the mating bearing surfaces, so that an air cushion for providing a low-friction bearing is established. In this known device, the magnets are provided within the disc outwardly of the hemispherical bearing members, and the mirror facets are again disposed outwardly of the magnets, so that the disc as a whole must have comparatively large radial dimensions and accordingly a large moment of inertia. This makes it difficult to control gyro-effects, when the laser printer is moved or shocks are applied thereto while the mirror is rotating. Further, the large radial dimensions of the disc result in correspondingly large centrifugal forces which, in view of the limited mechanical strength of the disc, impose an upper limit to the achievable rotary speed. On the other hand, a rotary speed as high as possible would be desirable in order to provide a high printing efficiency.
Another problem encountered with the above-described prior art relates to the so-called wobble, i.e. an undesired deflection of the reflected beam in the direction normal to the scanning direction. This wobble is caused by slight misalignments of the mirror facets due to manufacturing tolerances, and by the minor play in the air bearing which may lead to slight vibrations or oscillations of the rotating mirror body. Due to the comparatively large mass and moments of inertia of the mirror body, the frequency of these oscillations may be in the range of the rotational frequency of the mirror body, so that slight imbalances of the mirror body may lead to resonance problems.
In DE-A-2927199 a wobble-free rotary mirror system has been proposed in which, however, the light beam is not angularly deflected but is subject to an oscillating parallel displacement. In this device, an internal roof prism is eccentrically provided in one axial end surface of the rotating mirror body, the mirror facets being formed by the angled internal surfaces of this roof prism. The incoming light beam is aligned with the axis of rotation, and is subject to double-reflection at both mirror facets, such that the outgoing beam is again parallel with the incoming beam but radially outwardly offset relative thereto. Due to the rotation of the mirror body, the outgoing beam rotates about the incoming beam. One component of the two-dimensional rotating movement of the reflected beam is cancelled by a cylinder lens, so that an oscillating one-dimensional movement is achieved at the focus of the cylinder lens. This system is substantially wobble-free because the outgoing beam after double-reflection is always parallel to the incoming beam, irrespective of slight angular displacements of the axis of the mirror body. However, with this device, the amplitude of the oscillating movement of the reflected beam is not larger than the diameter of the mirror body. Accordingly, this system is not suited for obtaining large scanning widths.
In U.S. Pat. No. 4,662,709, a mirror system is proposed in which mirror facets are formed by the side surfaces of an internal frusto-pyramid formed in one axial end surface of the rotating mirror body. The incoming light beam is reflected at one of the mirror facets into the interior of the frusto-pyramid and undergoes two reflections at fixed mirror surfaces provided stationarily within the internal pyramid, so that the light is reflected back to the mirror facet where it is reflected for a second time. In this system, the mirror body must also have large radial dimensions, because the stationary mirrors have to be accommodated in the interior of the internal pyramid formed in the mirror body.
In U.S. Pat. No. 4,475,787 there is disclosed a single facet wobble-free light scanner which utilizes a pentaprism as rotating mirror body. The incoming light beam is aligned with the axis of rotation of the prism and enters through one surface of the prism which is normal to the axis of rotation. The light beam is then reflected at two inner surfaces of the prism and leaves the prism through another surface at right angles to the incoming beam. A problem involved in this structure is that the pentaprism is not symmetric with respect to the axis of rotation, so that it is difficult to balance the rotating mirror body accurately enough for permitting a high speed of revolution.